1. Technical Field
This invention relates generally to apparatus having a web drive mechanism for advancing web from a supply roll, and more particularly to such apparatus that returns some of the advanced web back toward the supply roll, wherein there is provided an non-motorized rewind mechanism for the supply roll to remove slack.
2. Background Art
The present invention is particularly useful in printer apparatus wherein a web of dye donor is advanced from a supply roll, past a thermal printhead, to a motorized take-up roll. Referring to FIG. 1, a commercially available thermal printer 10 includes a printhead assembly 12, dye donor web supply and take-up rolls 14 and 16, respectively, a roller platen assembly 18, a pair of pinch rollers 20 and 22, a dye receiver medium transport guide 30, and a dye receiver medium supply 24.
Normal thermal printer operations include loading dye receiver medium, printing information upon the dye receiver medium and ejecting the finished print. Each of these operations is fully described in commonly-assigned U.S. Pat. No. 5,176,458, which issued to H. G. Wirth on Jan. 5, 1993. Therefore only a brief description will be herein given of the illustrated embodiment of the thermal printer.
Printer operation begins with a loading phase, in which a sheet 28 of dye receiver medium advances from supply 24 along guide 30 to a gap between printhead assembly 12 and platen assembly 18. The leading edge of sheet 28 is held in the nip of rollers 20 and 22. Printhead assembly 12 moves toward platen assembly 18, pressing dye donor web 26 and the dye receiver medium against platen assembly 18 to form a sandwich for thermal printing.
Referring to FIG. 2, the printhead of printhead assembly 12 includes a plurality of heating elements 32, such as electrical resistors. When one of a plurality of switches 34 is closed, the associated heating element 32 is connected to a voltage potential source V.sub.S.
Dye donor web 26 comprises a leader portion followed by a repeating series of dye frames. The dye frames may be contiguous as shown or spaced by interframe regions, and, as shown in FIG. 3, each series includes in sequence yellow, magenta, and cyan dye frames. A single series is of course used to print one full-color image, comprising multiple color planes, on dye receiver medium 28.
As shown, there are two LEDs 36 and 38 which illuminate the dye donor web from above. LED 36 emits yellow light and LED 38 emits red light. Two photodetectors "A" and "B" are disposed below the dye donor web and receive light which passes through the dye donor web. Photodetectors "A" and "B" provide a signals for identifying the start of series and each individual color dye frame in such series. For a more complete discussion of this identification, reference is made to commonly assigned Reissue U.S. Pat. No. Re. 33,260 to S. Stephenson.
FIGS. 4 and 5 schematically illustrate the position of the dye donor web frames before and after LED 36 (schematically shown in these figures) have detected the transition between a used and an unused dye frame 40 and 42, respectively. If the printer is powered down in the position of FIG. 5, and then powered up while the web is in the position of FIG. 5, the printer logic cannot be sure where the frame transition is. Since the web drive is unidirectional (arrow 44), the printer must advance the web forward until it sees the next web frame transition into the first (yellow) frame. Doing so will waste one series of donor frames.